This invention relates to fluid operated piston-type actuators commonly referred to as fluid actuators. More particularly, this invention relates to a compact fluid actuated device having a cylindrical bore fitted with an axially reciprocable piston in a cylindrical bore, including means for controlling deceleration of the piston as it approaches the limit at each end of its travel within the cylindrical bore. Fluid, whether liquid or gas, to operate the device is supplied at a relatively low pressure in the range from about 1 to about 1000 psig, alternately to chambers on each side of the piston, and vented from the chambers, through ports in communication with the cylindrical bore. Fluid actuators of the type to which this invention is related are described in the art in U.S. Pat. Nos. 3,136,228; 3,303,756; 3,043,639; 2,673,130; and, 3,559,538 inter alia. The actuators are used to provide movement responsive to the application of hydraulic or pneumatic pressure alternately to one and then the other side of the reciprocable piston within the cylindrical bore. Typically, an operating shaft connected at one end to the piston and movable therewith, extends axially from the cylindrical bore and is drivingly engaged with a mechanism which benefits from the force exerted by the operating shaft. Normally, fluid actuators, particularly those in which the frequency of reciprocation of the piston is in the range from about 1 to about 100 cycles/min., are provided with some means for decelerating the piston as it nears the terminal portion of its stroke. These deceleration means, referred to as "snubbers" provide controlled deceleration to prevent damage associated with continual impact of the piston against the end walls of the fluid actuator. Snubbers may absorb impact by means of a fluid cushion which may be adjustable in relation to the energy to be absorbed; or, snubbers may be mechanical, relying on an elastomer material to absorb impact.
Deceleration is conventionally provided in a hydraulic cylinder by trapping a portion of the hydraulic fluid being discharged from the cylinder by the movement of the piston towards one sealed end of the cylinder, and causing the fluid to pass through a restricted orifice to convert the kinetic energy to heat.
As acknowledged in U.S. Pat. No. 3,559,538, deceleration means existing prior to this invention were complex in nature and required either separately formed passages in the operating cylinder, non-metallic seals exposed to substantial pressure differentials, assemblages of separate elements, or check valves in the discharge line. As a result, machining and assembly procedures were complex and functional weaknesses prevalent.
Moreover, in many prior art deceleration means, accurate control of the size of the restricting orifice, both during machining of the actuator components and during operation, was extremely difficult. As a result, desired deceleration rates were difficult to establish and maintain. A typical example of a solution to this problem is illustrated by the commonly used deceleration means which includes a buffer chamber associated with the operating cylinder, and a buffer piston assiciated with the movable piston. The diametrical clearance between the buffer chamber and buffer piston establishes the restricting orifice size, and rigid control of manufacturing and assembly tolerances is necessary to provide desired performance. In another design approach to solve this problem, each end wall or cylinder head of an operating cylinder is provided with annular interior and exerior grooves in which elastomer O-rings are held. The term "operating cylinder" is used herein to refer to the body of the device in which the cylindrical bore is provided, whether the body is a right cylinder or a rectangular parallelepiped. The operating shaft is slidably disposed within the interior O-ring which provides a cushion seal. The operating cylinder is also provided with a radial passage aligned with and in restrictedly open communication with the cushion seal by virtue of a recessed pressure adjusting means threadedly disposed in the operating cylinder head. Because each such conventional cylinder head is ported, and includes grooves for the O-rings and a groove for a circumflex square wire key, the head is necessarily thick, generally being about an inch thick, or more.
It is also noteworthy that in prior art fluid actuators in which an operating cylinder is provided with end walls retained by a square wire circumscribing the end wall, the wall of the operating cylinder is necessarily relatively thin. The cylinder wall is thin to allow the square wire to be threaded from outside the cylinder, through the wall and into an annular groove in the circumferential surface of each end wall. If the wall is thick, the square wire cannot be so threaded. The thin cylinder wall limits the pressure under which the cylinder may be operated. Further, as explained hereinabove, because most prior art end walls are necessarily at least of sufficient thickness to permit provision of a threaded port, the additional length of the overall device due to the thick end walls negates use of the device in many applications where compactness is essential, as is often the case with "clamping cylinders", and actuators for feeding small parts for assembly.
The present invention is a novel and surprisingly effective compact double-acting fluid actuated device having ports intermediate the end walls, which includes a deceleration means and thin end walls without the practical disadvantages of prior art devices. The deceleration means of the present invention does not require mechanical snubber means, check valves in the discharge conduit, separately formed elements associated with the piston or cylinder, or non-metallic seals exposed to extreme pressure differentials. It provides control of the piston deceleration rate while minimizing the complexity of providing and maintaining a restricted orifice flow path of desired size. By avoiding a mechanical snubber which is variably and unpredictably compressible, my compact fluid actuator provides a precise stroke in an operating cylinder which is only slightly longer than the length of its stroke. The actuator is termed "compact" because the overall length of the operating cylinder is determined by the thickness of the end walls selected for use under predetermined operating pressure rather than the diameter of the ports to be provided therein; and, the overall length is generally only slightly more than the length of the stroke.